The role of background selection in shaping patterns of molecular evolution and variation: evidence from variability on the Drosophila X chromosome

In the putatively ancestral population of Drosophila melanogaster, the ratio of silent DNA sequence diversity for X-linked loci to that for autosomal loci is approximately one, instead of the expected "null" value of 3/4. One possible explanation is that background selection (the hitchhiking effect of deleterious mutations) is more effective on the autosomes than on the X chromosome, because of the lack of crossing over in male Drosophila. The expected effects of background selection on neutral variability at sites in the middle of an X chromosome or an autosomal arm were calculated for different models of chromosome organization and methods of approximation, using current estimates of the deleterious mutation rate and distributions of the fitness effects of deleterious mutations. The robustness of the results to different distributions of fitness effects, dominance coefficients, mutation rates, mapping functions, and chromosome size was investigated. The predicted ratio of X-linked to autosomal variability is relatively insensitive to these variables, except for the mutation rate and map length. Provided that the deleterious mutation rate per genome is sufficiently large, it seems likely that background selection can account for the observed X to autosome ratio of variability in the ancestral population of D. melanogaster. The fact that this ratio is much less than one in D. pseudoobscura is also consistent with the model's predictions, since this species has a high rate of crossing over. The results suggest that background selection may play a major role in shaping patterns of molecular evolution and variation.     

Polymorphism and divergence at the prune locus in Drosophila melanogaster and D. simulans

The prune locus of Drosophila melanogaster lies at the tip of the X chromosome, in a region of reduced recombination in which nearby loci show reduced variation relative to evolutionary divergence from D. simulans. DNA sequencing of prune alleles from D. melanogaster and D. simulans reveals extremely low variation in D. melanogaster but greater variation in D. simulans. Divergence between the two species is not reduced. This pattern may be explained by either positive selection leading to hitchhiking of neutral variation or background selection against deleterious mutations. The pattern of silent versus replacement polymorphism and divergence at prune is consistent with either a model of weakly deleterious selection against amino acid substitutions or balancing selection.      

High nucleotide sequence variation in a region of low recombination in Drosophila simulans is consistent with the background selection model

We surveyed nucleotide sequence variation at glucose dehydrogenase (Gld), in a region of low recombination on chromosome 3R, from a population sample of Drosophila simulans. The levels of nucleotide variation were surprisingly high. There was no departure from the expectation of a neutral model for the level of polymorphism, indicating no evidence of a selective sweep in this region. There was a significant deficiency of singleton polymorphisms according to the Fu and Li test, although Tajima and Hudson, Kreitman, and Aguade (HKA) tests do not provide evidence of a significant elevation of variation due to balancing selection. Genetic map data for the D. simulans third chromosome were used to calculate expected values of pi for Gld under a current model of background selection, varying the values for the parameter sh (selection coefficient against deleterious mutations). We show that the recombinational landscape of D. simulans is sufficiently different from that of D. melanogaster that we expect higher variation under the background selection model, even when effective population sizes are assumed to be equal. The data for Gld were tested against the predictions using computer simulations of the distribution of the number of segregating sites conditioned on pi. Background selection alone can explain our observations as long as sh is larger than 0.005 and species-level effective population size is assumed to be several- fold larger than in D. melanogaster. Alternatively, the deleterious mutation rate may be smaller in D. simulans, or balancing selection may be acting nearby, thereby reducing the effect of background selection.      

Reduced natural selection associated with low recombination in Drosophila melanogaster

Synonymous codons are not used equally in many organisms, and the extent of codon bias varies among loci. Earlier studies have suggested that more highly expressed loci in Drosophila melanogaster are more biased, consistent with findings from several prokaryotes and unicellular eukaryotes that codon bias is partly due to natural selection for translational efficiency. We link this model of varying selection intensity to the population-genetics prediction that the effectiveness of natural selection is decreased under reduced recombination. In analyses of 385 D. melanogaster loci, we find that codon bias is reduced in regions of low recombination (i.e., near centromeres and telomeres and on the fourth chromosome). The effect does not appear to be a linear function of recombination rate; rather, it seems limited to regions with the very lowest levels of recombination. The large majority of the genome apparently experiences recombination at a sufficiently high rate for effective natural selection against suboptimal codons. These findings support models of the Hill-Robertson effect and genetic hitchhiking and are largely consistent with multiple reports of low levels of DNA sequence variation in regions of low recombination.      

DNA sequence variation and the recombinational landscape in Drosophila pseudoobscura: a study of the second chromosome.

The relationship between rates of recombination and DNA sequence polymorphism was analyzed for the second chromosome of Drosophila pseudoobscura. We constructed integrated genetic and physical maps of this chromosome using molecular markers at 10 loci spanning most of its physical length. The total length of the map was 128.2 cM, almost twice that of the homologous chromosome arm (3R) in D. melanogaster. There appears to be very little centromeric suppression of recombination, and rates of recombination are quite uniform across most of the chromosome. Levels of sequence variation (theta(W), based on the number of segregating sites) at seven loci (tropomyosin 1, Rhodopsin 3, Rhodopsin 1, bicoid, Xanthine dehydrogenase, Myosin light chain 1, and ribosomal protein 49) varied from 0.0036 to 0.0167. Generally consistent with earlier studies, the average estimate of theta(W) at total sites is 1.5-fold higher than that in D. melanogaster, while average theta(W) at silent sites is almost 3-fold higher. These estimates of variation were analyzed in the context of a background selection model under the same parameters of mutation rate and selection as have been proposed for D. melanogaster. It is likely that a significant fraction of the higher level of sequence variation in D. pseudoobscura can be explained by differences in regional rates of recombination rather than a larger species-level effective population size. However, the distribution of variation among synonymous, nonsynonymous, and noncoding sites appears to be quite different between the species, making direct comparisons of neutral variation, and hence inferences about effective population size, difficult. Tajima's D statistics for 6 out of the 7 loci surveyed are negative, suggesting that D. pseudoobscura may have experienced a rapid population expansion in the recent past or, alternatively, that slightly deleterious mutations constitute an important component of standing variation in this species.     

Nonneutral admixture of immigrant genotypes in African Drosophila melanogaster populations from Zimbabwe

Drosophila melanogaster originated in Africa and colonized the rest of the world only recently (approximately 10,000 to 15,000 years ago). Using 151 microsatellite loci, we investigated patterns of gene flow between African D. melanogaster populations representing presumptive ancestral variation and recently colonized European populations. Although we detected almost no evidence for alleles of non-African ancestry in a rural D. melanogaster population from Zimbabwe, an urban population from Zimbabwe showed evidence for admixture. Interestingly, the degree of admixture differed among chromosomes. X chromosomes of both rural and urban populations showed almost no non-African ancestry, but the third chromosome in the urban population showed up to 70% of non-African alleles. When chromosomes were broken into contingent microsatellite blocks, even higher estimates of admixture and significant heterogeneity in admixture was observed among these blocks. The discrepancy between the X chromosome and the third chromosome is not consistent with a neutral admixture hypothesis. The higher number of European alleles on the third chromosome could be due to stronger selection against foreign alleles on the X chromosome or to more introgression of (beneficial) alleles on the third chromosome.     

S-element insertions are associated with the evolution of the Hsp70 genes in Drosophila melanogaster

The "selfish DNA" theory postulates that transposable elements (TEs) are intragenomic parasites, and that natural selection against deleterious effects associated with their presence is the main force preventing their genomic spread in natural populations. In agreement with this model, TEs in Drosophila melanogaster populations are usually found at low frequencies in most genomic locations. Only a few cases of fixation of TE insertions have been reported, usually in regions of low recombination, where selection is expected to be less effective. Here, we report a population genetics study on the apparent fixation of an S-element in a highly recombining region in two natural populations of D. melanogaster. Three similar fragments of an S-element are inserted into the 5' regions of three members of a heat shock gene family, Hsp70 (Hsp70Aa and Hsp70Ab in polytene chromosome band 87A, and Hsp70Bb in 87C). A PCR-based analysis suggests that the insertions are fixed or at high frequencies in the entire species. A population survey of the levels of nucleotide sequence variation at the insertion site in 87C in two natural populations of D. melanogaster provided evidence for reduced levels of variation in the region, normal levels of recombination, and selection, reflected in a significant departure from neutrality of the variant frequency spectrum. This was particularly strong for the S-element inverted repeats (IRs) and suggests that these are of functional significance for the host.     

High DNA sequence diversity in pericentromeric genes of the plant Arabidopsis lyrata

Differences in neutral diversity at different loci are predicted to arise due to differences in mutation rates and from the "hitchhiking" effects of natural selection. Consistent with hitchhiking models, Drosophila melanogaster chromosome regions with very low recombination have unusually low nucleotide diversity. We compared levels of diversity from five pericentromeric regions with regions of normal recombination in Arabidopsis lyrata, an outcrossing close relative of the highly selfing A. thaliana. In contrast with the accepted theoretical prediction, and the pattern in Drosophila, we found generally high diversity in pericentromeric genes, which is consistent with the observation in A. thaliana. Our data rule out balancing selection in the pericentromeric regions, suggesting that hitchhiking is more strongly reducing diversity in the chromosome arms than the pericentromere regions.     

Estimates of the genomic mutation rate for detrimental alleles in Drosophila melanogaster

The net rate of mutation to deleterious but nonlethal alleles and the sizes of effects of these mutations are of great significance for many evolutionary questions. Here we describe three replicate experiments in which mutations have been accumulated on chromosome 3 of Drosophila melanogaster by means of single-male backcrosses of heterozygotes for a wild-type third chromosome. Egg-to-adult viability was assayed for nonlethal homozygous chromosomes. The rates of decline in mean and increase in variance (DM and DV, respectively) were estimated. Scaled up to the diploid whole genome, the mean DM for homozygous detrimental mutations over the three experiments was between 0.8 and 1.8%. The corresponding DV estimate was approximately 0.11%. Overall, the results suggest a lower bound estimate of at least 12% for the diploid per genome mutation rate for detrimentals. The upper bound estimates for the mean selection coefficient were between 2 and 10%, depending on the method used. Mutations with selection coefficients of at least a few percent must be the major contributors to the effects detected here and are likely to be caused mostly by transposable element insertions or indels.     

Evidence of susceptibility and resistance to cryptic X-linked meiotic drive in natural populations of Drosophila melanogaster

There is mounting evidence consistent with a general role of positive selection acting on the Drosophila melanogaster X-chromosome. However, this positive selection need not necessarily arise from forces that are adaptive to the organism. Nonadaptive meiotic drive may exist on the X-chromosome and contribute to forces of selection. Females from a reference D. melanogaster line, containing the X-linked marker white, were crossed to males from 49 isofemale lines established from seven African and five non-African natural populations to detect naturally occurring meiotic drive. Several lines exhibited a departure from expected Mendelian transmission of X-chromosomes to the third generation (F2) offspring, particularly those from hybrid African male parents. F2 viability was not correlated with skewed chromosomal inheritance. However, a significant difference in viability between cosmopolitan and tropical African crosses was observed. Recombination analysis supports the presence of a male-acting meiotic drive element near the centromeric region of the X-chromosome and putative recessive autosomal drive suppression. There is also evidence of another female-acting drive element linked to white. The possible role meiotic drive may contribute in shaping levels of genetic variation in D. melanogaster, and additional ways to test this hypothesis are discussed.     

Nucleotide variation and recombination along the fourth chromosome in Drosophila simulans

The fourth chromosome of Drosophila melanogaster and its sister species are believed to be nonrecombining and have been a model system for testing predictions of the effects of selection on linked, neutral variation. We recently examined nucleotide variation along the chromosome of D. melanogaster and revealed that a low average level of recombination could be associated with considerably high levels of nucleotide variation. In this report, we further investigate the variation along the fourth chromosome of D. simulans. We sequenced 12 gene regions evenly distributed along the fourth chromosome for a worldwide collection of 11 isofemale lines and 5 gene regions in a local population of 10 isofemale lines from South America. In contrast to predictions for regions of very low recombination, these data reveal that the variation levels in many gene regions, including an intron region of the ci gene, vary considerably along the fourth chromosome. Nucleotide diversity ranged from 0.0010 to 0.0074 in 9 gene regions interspersed with several regions of greatly reduced variation. Tests of recombination indicate that the recombination level is not as low as previously thought, likely an order of magnitude higher than that in D. melanogaster. Finally, estimates of the recombination parameters are shown to support a crossover-plus-conversion model.     

The rate of adaptation in asexuals

I study the population genetics of adaptation in asexuals. I show that the rate of adaptive substitution in an asexual species or nonrecombining chromosome region is a bell-shaped function of the mutation rate: at some point, increasing the mutation rate decreases the rate of substitution. Curiously, the mutation rate that maximizes the rate of adaptation depends solely on the strength of selection against deleterious mutations. In particular, adaptation is fastest when the genomic rate of mutation, U, equals the harmonic mean of selection coefficients against deleterious mutations, where we assume that selection for favorable alleles is milder than that against deleterious ones. This simple result is independent of the shape of the distribution of effects among favorable and deleterious mutations, population size, and the action of clonal interference. In the course of this work, I derive an approximation to the probability of fixation of a favorable mutation in an asexual genome or nonrecombining chromosome region in which both favorable and deleterious mutations occur.     

Linkage disequilibrium patterns across a recombination gradient in African Drosophila melanogaster

Previous multilocus surveys of nucleotide polymorphism have documented a genome-wide excess of intralocus linkage disequilibrium (LD) in Drosophila melanogaster and D. simulans relative to expectations based on estimated mutation and recombination rates and observed levels of diversity. These studies examined patterns of variation from predominantly non-African populations that are thought to have recently expanded their ranges from central Africa. Here, we analyze polymorphism data from a Zimbabwean population of D. melanogaster, which is likely to be closer to the standard population model assumptions of a large population with constant size. Unlike previous studies, we find that levels of LD are roughly compatible with expectations based on estimated rates of crossing over. Further, a detailed examination of genes in different recombination environments suggests that markers near the telomere of the X chromosome show considerably less linkage disequilibrium than predicted by rates of crossing over, suggesting appreciable levels of exchange due to gene conversion. Assuming that these populations are near mutation-drift equilibrium, our results are most consistent with a model that posits heterogeneity in levels of exchange due to gene conversion across the X chromosome, with gene conversion being a minor determinant of LD levels in regions of high crossing over. Alternatively, if levels of exchange due to gene conversion are not negligible in regions of high crossing over, our results suggest a marked departure from mutation-drift equilibrium (i.e., toward an excess of LD) in this Zimbabwean population. Our results also have implications for the dynamics of weakly selected mutations in regions of reduced crossing over.     

Recombination, dominance and selection on amino acid polymorphism in the Drosophila genome: contrasting patterns on the X and fourth chromosomes

Surveys of nucleotide polymorphism and divergence indicate that the average selection coefficient on Drosophila proteins is weakly positive. Similar surveys in mitochondrial genomes and in the selfing plant Arabidopsis show that weak negative selection has operated. These differences have been attributed to the low recombination environment of mtDNA and Arabidopsis that has hindered adaptive evolution through the interference effects of linkage. We test this hypothesis with new sequence surveys of proteins lying in low recombination regions of the Drosophila genome. We surveyed >3800 bp across four proteins at the tip of the X chromosome and >3600 bp across four proteins on the fourth chromosome in 24 strains of D. melanogaster and 5 strains of D. simulans. This design seeks to study the interaction of selection and linkage by comparing silent and replacement variation in semihaploid (X chromosome) and diploid (fourth chromosome) environments lying in regions of low recombination. While the data do indicate very low rates of exchange, all four gametic phases were observed both at the tip of the X and across the fourth chromosome. Silent variation is very low at the tip of the X (thetaS = 0.0015) and on the fourth chromosome (thetaS = 0.0002), but the tip of the X shows a greater proportional loss of variation than the fourth shows relative to normal-recombination regions. In contrast, replacement polymorphism at the tip of the X is not reduced (thetaR = 0.00065, very close to the X chromosome average). MK and HKA tests both indicate a significant excess of amino acid polymorphism at the tip of the X relative to the fourth. Selection is significantly negative at the tip of the X (Nes = -1.53) and nonsignificantly positive on the fourth (Nes approximately 2.9), analogous to the difference between mtDNA (or Arabidopsis) and the Drosophila genome average. Our distal X data are distinct from regions of normal recombination where the X shows a deficiency of amino acid polymorphism relative to the autosomes, suggesting more efficient selection against recessive deleterious replacement mutations. We suggest that the excess amino acid polymorphism on the distal X relative to the fourth chromosome is due to (1) differences in the mutation rate for selected mutations on the distal X or (2) a greater relaxation of selection from stronger linkage-related interference effects on the distal X. This relaxation of selection is presumed to be greater in magnitude than the difference in efficiency of selection between X-linked vs. autosomal selection.     

Molecular Population Genetics of the Distal Portion of the X Chromosome in Drosophila: Evidence for Genetic Hitchhiking of the Yellow-Achaete Region

We have estimated DNA sequence variation and differentiation within and between Drosophila melanogaster and its sibling species, Drosophila simulans, using six-cutter restriction site variation at yellow-achaete (y-ac), phosphogluconate dehydrogenase (Pgd), and period (per). These three gene regions are of varying distance from the telomere of the X chromosome and range from very low to moderate rates of recombination in D. melanogaster. According to Tajima's test of neutrality, the Pgd region has been influenced by balancing selection in D. melanogaster. This is consistent with previous data suggesting the allozyme polymorphism at this locus is visible to selection. The Hudson, Kreitman, Aguadé test of neutrality reveals a significant departure from neutrality for the y-ac region compared to the per or rosy regions in D. simulans. There is also a significant departure for the y-ac region compared to the Adh 5' flanking region in D. melanogaster. In both species the departure appears to be due to reduced variation at y-ac compared to that expected from divergence between D. simulans and D. melanogaster. We conclude that recent hitchhiking associated with the selective fixation of one or more advantageous mutants in the y-ac region is the best explanation for reduced variation at y-ac.     

Mutant alleles of small effect are primarily responsible for the loss of fitness with slow inbreeding in Drosophila melanogaster.

Multilocus simulation is used to identify genetic models that can account for the observed rates of inbreeding and fitness decline in laboratory populations of Drosophila melanogaster. The experimental populations were maintained under crowded conditions for approximately 200 generations at a harmonic mean population size of Nh approximately 65-70. With a simulated population size of N = 50, and a mean selective disadvantage of homozygotes at individual loci approximately 1-2% or less, it is demonstrated that the mean effective population size over a 200-generation period may be considerably greater than N, with a ratio matching the experimental estimate of Ne/Nh approximately 1.4. The buildup of associative overdominance at electrophoretic marker loci is largely responsible for the stability of gene frequencies and the observed reduction in the rate of inbreeding, with apparent selection coefficients in favor of the heterozygote at neutral marker loci increasing rapidly over the first N generations of inbreeding to values approximately 5-10%. The observed decline in fitness under competitive conditions in populations of size approximately 50 in D. melanogaster therefore primarily results from mutant alleles with mean effects on fitness as homozygotes of sm < or = 0.02. Models with deleterious recessive mutants at the background loci require that the mean selection coefficient against heterozygotes is at most hsm approximately 0.002, with a minimum mutation rate for a single Drosophila autosome 100 cM in length estimated to be in the range 0.05-0.25, assuming an exponential distribution of s. A typical chromosome would be expected to carry at least 100-200 such mutant alleles contributing to the decline in competitive fitness with slow inbreeding.     

Regulation of genome stability by TEL1 and MEC1, yeast homologs of the mammalian ATM and ATR genes

DNA sequence surveys of Drosophila melanogaster populations show a strong positive correlation between the recombination rate experienced by a locus and its level of nucleotide polymorphism. In particular, surveys of the fourth chromosome gene ci(D) show greatly reduced levels of nucleotide variation; this observation was originally interpreted in terms of selective sweeps occurring on the nonrecombining fourth chromosome. Subsequent theoretical work has, however, uncovered several other selective processes that can reduce variation. In this study, we revisit the Drosophila fourth chromosome, investigating variation in 5-6 kb of the gene ankyrin in D. melanogaster and D. simulans. Silent nucleotide site diversity is approximately 5 x 10(-4) for both species, consistent with the previous observations of low variation at ci(D). Given the observed frequency spectra at ankyrin, coalescent simulations indicate that reduced diversity in the region is unlikely to be due to a selective sweep alone. We find evidence for recombinational exchange at this locus, and both species appear to be fixed for an insertion of the transposable element HB in an intron of ankyrin.     

A scan of molecular variation leads to the narrow localization of a selective sweep affecting both Afrotropical and cosmopolitan populations of Drosophila melanogaster

Drosophila melanogaster originated in tropical Africa but has achieved a cosmopolitan distribution in association with human habitation. Cosmopolitan populations of D. melanogaster are known to have reduced genetic variation, particularly on the X chromosome. However, the relative importance of population bottlenecks and selective sweeps in explaining this reduction is uncertain. We surveyed variation at 31 microsatellites across a 330-kb section of the X chromosome located between the white and kirre genes. Two linked clusters of loci were observed with reduced variation and a skew toward rare alleles in both an Ecuador and a Zimbabwe population sample. Examining Zimbabwe DNA sequence polymorphism within one of these regions allowed us to localize a selective sweep to a 361-bp window within the 5' regulatory region of the roughest gene, with one nucleotide substitution representing the best candidate for the target of selection. Estimates of sweep age suggested that this fixation event occurred prior to the expansion of D. melanogaster from sub-Saharan Africa. For both putative sweep regions in our data set, cosmopolitan populations showed wider footprints of selection compared to those in Zimbabwe. This pattern appears consistent with the demographic amplification of preexisting sweep signals due to one or more population bottlenecks.